Abstract

Autonomous navigation through unknown environments is a challenging task that entails real-time localization, perception, planning, and control. UAVâs with this capability have begun to emerge in the literature with advances in lightweight sensing and computing. Although the planning methodologies vary from platform to platform, many algorithms adopt a hierarchical planning architecture where a slow, low-fidelity global planner guides a fast, high-fidelity local planner. However, in unknown environments, this approach can lead to erratic or unstable behavior due to the interaction between the global planner, whose solution is changing constantly, and the local planner; a consequence of not capturing higher-order dynamics in the global plan. This work proposes a planning framework in which multi-fidelity models are used to reduce the discrepancy between the local and global planner. Our approach uses high-, medium-, and low-fidelity models to compose a path that captures higher-order dynamics while remaining computationally tractable. In addition, we address the interaction between a fast planner and a slower mapper by considering the sensor data not yet fused into the map during the collision check. This novel mapping and planning framework for agile flights is validated in simulation, showing replanning times of 5-40 ms in cluttered environments, a value that is 3-30 times faster than similar state-of-the-art planning algorithms.

I Introduction

UAV autonomous navigation in unknown environments has received special interest in the last few years because of its unlimited applications, ranging from aerial surveying and inspection to search and rescue. However, these applications are often reduced to low-speed flights due to the current limitations and low rates of the state-of-the-art mappers and planners. The inherent non-convexity of the path planning optimization problem, together with the high mapping and planning rate needed for agile flights make this problem especially hard. This work presents a novel framework to perform high-rate mapping and planning in unknown environments suitable for agile maneuvers, addressing the fundamental problem between the interaction of a global planner and a local planner.

Computational tractability of the planning problem leads to the use of a low-fidelity global planner that computes a cost-to-go (CTG) needed by the high-fidelity local planner. However, the fact that the global planner does not account for the dynamics results in erratic behaviors when the world model is changing rapidly. There is therefore a need of an accurate CTG calculation that captures both the global environment and the dynamic feasibility, maintaining relatively low computation times at the same time.

Fig. 1: Global optimum and our method. When the map is completely known, the optimal trajectory computed using the approach of [1] is shown in blue (). The red trajectory () is the solution found by our method, where the world is not known and it is being discovered as the UAV flies forward. The grid is 1m ×1m, and the sensing range is 10 m.

Moreover, the choice of the representation of the environment and the size of the “global” map (larger scale than the sensor FOV and the local representation, but typically does not contain all information observed to reduce effort) have a significant impact on the computational cost, but for most systems updates of these models cannot be done at the sensor frame rates (∼30 Hz) and updates are typically slower than the re-plan rate. Thus a second design challenge is how to combine the global knowledge (available at a slower rate) with the high-rate local information in the planner representation of the environment. Finally, the state-of-the-art mappers and planners run onboard at ∼5 Hz, so the the third key challenge is how to optimize the planning and mapping algorithms to achieve higher rates, suitable for aggressive flights.

This work addresses these challenges with the following contributions:

A novel formulation of the planning problem that takes into account the dynamics of the vehicle in the cost-to-go calculation to solve the negative interaction that usually occurs between the global and local planners when operating in unknown environments.

A lightweight fused-based mapping framework using a sliding map to reduce the estimation error influence that runs onboard fusing a depth image in 50 ms.

An integration of a high-rate planner with a slower-rate mapper, with a collision check algorithm that accounts for both the most recent fused information and the available sensed data not included in that map.

I-a Hardware Experiments

The UAV used for the future hardware experiments is shown in Fig. 2. All the perception, planning and control runs onboard, and the position, velocity, attitude, and IMU biases are estimated by fusing propagated IMU measurements with an external motion capture system via a Kalman filter. The mapping fusion times achieved onboard are 50ms and 80ms for depth image resolutions of 480×270 and 640×480 respectively.
All these experiments are available in this link and also in the video accompanying this work.

Fig. 2: UAV used in the experiments. It is equipped with a Qualcomm® SnapDragon Flight, an Nvidia® Jetson TX2 and an Intel® RealSense Depth Camera D435.

Ii Conclusions

This work presented a novel planning a mapping framework suitable for agile flights in unknown environments. The key properties of this framework is its ability to solve the interaction between the global planner and the local planner considering the dynamics of the vehicle, and its ability to address efficiently the integration between a fast planner and a slower mapper. The replanning and mapping rates are several times faster than the state of the art.

Acknowledgment

Thanks to Boeing Research & Technology for support of
the hardware, to Helen Oleynikova (ASL-ETH) for the data of the forest simulation, and to Pablo Tordesillas (ETSAM-UPM) for his help with some figures of this paper. Supported in part by Defense Advanced Research Projects Agency (DARPA) as part of the Fast Lightweight Autonomy (FLA) program, HR0011-15-C-0110.
Views expressed here are those of the authors, and do not reflect the official views or policies of the Dept. of Defense or the U.S. Government.

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